1. Field of the Invention.
This invention relates to a carrier plate for use in a lamination process in the preparation of a multicolor proof.
2. Description of Related Art.
Reproduction of color images through printing is an involved process requiring a breakdown of an original image into a number of fundamental single color images and the eventual recombination of the single color images through multiple printings into a full color reproduction of the original. Color image reproduction typically includes the following steps.
First, using filtering and other photographic techniques a number of color separation transparencies or color separations are produced, typically four, each bearing a halftone dot and/or a continuous tone (contone) image corresponding, for instance, to magenta, cyan, yellow and black portions of the original. Second, a printing plate is made for each color separation by exposing a sensitized plate through one of the transparencies and developing the plate. Third, the printing plates are mounted on a multistage printing press which prints inks on a substrate, typically paper, sequentially and in register, one on top of another, four halftone and/or contone images corresponding to each of the color separations to create a full color reproduction of the original.
In the case of halftone images, the various colors are reproduced by the superposition of a multiplicity of dots of varying diameters and colors corresponding to the four color separations. Any deviation in the size or position of the printed dots results in color shifts in the final printed image. As a result of this reproduction process, setting up the press and printing a four color image is economically feasible typically only when employed for printing large quantities of a given original.
It is desirable to be able to predict the final image appearance before it is printed, and preferably before making printing plates by reviewing images made directly from the color separation transparencies. The art of evaluating the color separation transparencies and deciding whether the various colors have indeed been properly separated is called proofing. Proofing is a process which uses the color separations to create a colored image called a proof to visualize what the final printed image will look like typically without actually making printing plates and running the printing press. Proofing through the creation of proofs shows the printer and customer what the job will look like after printing, so changes can be made, if necessary, to the color separations before the job goes to press where it can waste expensive press time, paper and ink if it is not right. Proofs are used for many purposes including for customer approval, for checking compatibility of a number of subjects to be printed on the same plate or job, for internal quality checking and control, and for relatively small numbers of final reproductions.
For many years the only way to make proofs was to print them on a press. This involved making plates, mounting them on the press, making the press ready to run, and running a few prints. Proofs of this type are very expensive because they involve labor intensive operations and the use of expensive materials (e.g., plates) and cost intensive equipment (e.g., the press). Special proof presses have been built to eliminate the high costs of using production presses, but manpower costs are still high and special proof presses do not always reproduce the printing and visual characteristics of the production press. Furthermore, press proofing by either of these techniques takes a long time to make proofs.
Because of the time and expense to make press proofs attempts have been made to develop and use less expensive and faster alternatives to press proofs. These are usually made by photochemical or photomechanical means and are referred to as off-press, pre-press or pre-plate proofs.
Photochemical or photomechanical proofing systems typically use photosensitive elements in making proofs. These systems generally make proofs by exposing photosensitive elements to actinic radiation through one of the image bearing color separation transparencies to produce a duplicate image that is either a positive or a negative of the transparency depending on the element used. The radiation may make soluble areas insoluble, insoluble areas soluble, tacky areas non tacky, or non tacky areas tacky depending on the element used. After imagewise exposure, the photosensitive element can be developed by washing out soluble areas. Then tacky areas of the element may have to be toned with a dry or liquid colorant. This process is repeated for all color separations. Then the processed elements are laminated together one at a time sometimes on a support or substrate. Protective layers may be peeled apart and removed from the elements before they are laminated to the support or other image elements. Finally, the four color images may be transferred from the support to a receptor, transfer or display sheet, such as a sheet of paper, to form the final proof.
Many photosensitive elements used for image reproduction have the disadvantage that they must be washed out by an organic solvent or aqueous base. For instance, U.S. Pat. No. 4,053,313 to Fan discloses a photosensitive element comprising a solvent-processable photosensitive layer, a tonable elastomeric layer and a support, that is processed (i.e., washed-out) by a mixture of water and organic solvent. The elements disclosed in U.S. Pat. No. 4,376,158 and 4,376,159 to Speckler require aqueous base for washout. Precolored diazo based photosensitive elements which are developed in a mixture of 1-propanol and water are disclosed, for example, in U.S. Pat. No. 3,671,236 to Van Beusekom, U.S. Pat. No. 4,656,114 to Cederburg and U.S. Pat. No. 4,666,817 to Sachi. Use of organic solvents or aqueous bases frequently is undesirable due to flammability, toxicity, corrosion and/or waste disposal considerations. The photosensitive elements disclosed in these Fan and Speckler Patents also require the developed (i.e., washed-out) elements to be toned either manually or in a toning apparatus.
Photosensitive elements are known and are being developed that do not have the above described disadvantages. Specifically, U.K. Patent publication GB 2144867 B discloses photosensitive elements having entirely aqueously developable precolored layers containing a colorant on a removable support. The U.K. Patent further describes a photomechanical process for forming a multicolor image, suitable for use as an off-press proof utilizing the aqueously developable photosensitive elements. The process for preparing a multicolor image is accomplished by (1) imagewise exposure of a first one of the precolored photosensitive elements comprising an aqueously developable precolored photosensitive layer on a removable support through a first one of the color separation transparencies forming image areas and non-image areas in its aqueously developable precolored layer, (2) processing the exposed element to develop the color image by washing away the non-image areas with water, (3) laminating the image bearing element to a substrate, and then (4) peeling the removable support from the image bearing element. A second color image element is made by exposing a second precolored photosensitive element with a second one of the color separation transparencies, processing to develop the color image, laminating in register the second color image element to the first color image element on the substrate and peeling the removable support from the second color image element. By repeating the exposing, processing, laminating in register, and peeling steps two more times, a four color image can be obtained. As long as the emulsion layer of the color separation contacts the photosensitive layer in the photosensitive element during the exposure step as is preferred, the process described in this U.K. Patent results in a wrong reading proof on the substrate. This means that the image on the substrate is a mirror image of that captured in the color separations.
Often it is desirable to view the multicolored image on a receptor or a transfer sheet, such as paper, which is the same as or similar to that which will be used in the final printing run. In this case, the multicolor image is transferred to the desired receptor by laminating and then peeling away the substrate. This results in a right reading proof assuming the emulsion layer of the color separation contacts the photosensitive layer in the photosensitive element during the exposure step. A right reading proof is the same as the original image, not a mirror image.
Many apparatuses have been developed for laminating photosensitive elements. Different laminating apparatuses are designed and used for laminating different photosensitive elements. However, aqueously developable precolored photosensitive elements are a relatively recent development. As such, few apparatuses exist for use in laminating aqueously developed precolored photosensitive elements or image sheets to supports or receiving substrates.
U.S. Pat. No. 4,659,927 to Tago et al. assigned to Fuji Photo Film Co., Ltd., discloses a support plate, a laminator, and a method utilizing the laminator for preparing a multicolor proof. The support plate is described as a thin plate of about 0.5 mm thickness, with good thermal conductivity and rigidity. The material suggested for the support plate is aluminum. The surface of the support plate is provided with a white layer of anodized aluminum for allowing positional adjustment of image elements. The support plate includes a cover sheet. The cover sheet is preferably made of plastics such as polyethylene terephthalate, but is not restricted to plastics and can include a thin metal sheet. The surface of the cover sheet may be coated with silicone or a fluorine-containing resin to prevent adhesion of the cover sheet to an image sheet. An assembly for transfer in the laminator is, in order, the support plate, the image-receiving sheet, the image sheet, and the cover sheet. The cover sheet presses the image sheet and the image-receiving sheet down against the support plate. Since the support plate is rigid, the assembly is prevented from deformation when pressed by heating rollers, and the image sheet and the image receiving sheet are prevented from positional displacement.
A support plate, commercially available from Fuji Photo Film Co., Ltd., is used for laminating aqueously developed precolored photosensitive elements to receiving substrates. Referring to FIGS. 1A and 1B, the support plate 100 of the prior art includes the parts described in the aforedescribed Fuji Patent. The support plate 100 is an aluminum metal support 101, 0.020 in [0.51 mm] thick, having a top side 102 and a bottom side 104. The top side 102 has a first layer 106 of titanium dioxide on the plate 100 and a second layer 108 of a clear polyurethane on the first layer 106. The cover sheet 110 resides on the top side 102 and is hingedly mounted with tape 111 to a front edge 112 of the support plate 100. The bottom side 104 of the support plate 100 has a third layer 114 of clear epoxy.
A particular problem of the laminate-in-register process is in laminating a processed image carrying film to the receiving substrate without causing any image distortion. The image distortion is observed when the dots of the subsequent color image carrying film laminations do not lay in proper position to a previous color image layer's dot. The dimensional error due to the misalignment or mis-registration of each color image layer's dot is referred to as registration error. Registration error becomes noticeable as a color shift when dots are mis-registered by more than half the diameter of a dot. In commercial 200 line per inch printing this maximum registration error translates to 63 microns.
A contributor to registration error can be from the dimensional instability of the polyester base used in each of the color image element and the receiving substrate. Inherently polyester base changes dimensions, i.e., expands, when heated but eventually returns close to its original dimensions when returned to room temperature. The thermal expansion properties of a base are governed by the thermal history that the base underwent during its manufacture. If the thermal histories of the polyester bases for the image carrying elements and the receiving substrate are different, the bases will expand differently upon heating during lamination.
The dimensional instability of the bases complicates registration of the image since, during lamination, the color image of the color image element transfers from the element to the receiving substrate. The color image in the image element resides on the expanding base and expands along with the base. The receiving substrate has an adhesive polymer which expands with the base of the substrate. During the elevated temperature condition of lamination, while both the substrate and the element are expanded, but to different degrees, the image layer is transferred to the receiving substrate. When the element and the substrate cool to room temperature, the image layer cools down while adhered to the base of the substrate. Thus, the position of the dots of the color image, i.e., registration, is influenced by the dimensional changes that occurs for each base. Since the color image transfer process repeats itself 4 times, the final four color image can be grossly out of registration.
It is recognized that the differences in dimensional stability of the bases for the color image element and the receiving substrate can be compensated for by using bases with the same thermal history for the element and substrate, or by using isotropic base. But these alternatives may not be economically or operationally feasible. The difference in dimensional stability of the bases could also be compensated for by a lower lamination temperature, i.e., lower thermal energy applied during lamination. In this case however, the color image typically will not adhere sufficiently to the receiving substrate after lamination.
A need exists to laminate a color image element to a receiving substrate with improved registration of resulting proofs. Particularly, this need exists in off-press apparatus for laminating aqueously developed precolored photosensitive elements or image sheets to supports or receiving substrates using a carrier plate in the process.